Silicon sputtering target with special surface treatment and good particle performance and methods of making the same

ABSTRACT

A sputter target assembly comprising a Si target and a backing plate is provided wherein the backing plate is bonded to the target. The Si target comprises a smooth, mirror-like surface and has a surface roughness of less than about 15.0 Angstroms. Methods are provided for producing silicon target/backing plate assemblies wherein a silicon blank is processed to remove scratches from the blank surface resulting in a mirror like surface on the target, and a surface roughness of 15.0 Angstroms or less. The method comprises a first and second cleaning step with the first step being performed before the scratch removal step, and the second step being performed after the scratch removal.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of U.S. Provisional PatentApplication Ser. No. 61/556,926 filed Nov. 8, 2011.

FIELD OF INVENTION

The invention relates to a silicon sputtering target bonded to a backingplate consisting of Mo, Ti, Zr, Ta, Hf, Nb, W, Cu and the alloys ofthese elements or Mo/Cu, Ti/Al composite backing plates, and to a methodof producing such a target. One aspect of the invention relates to asilicon-backing plate sputtering target assembly with a special surfacetreatment.

BACKGROUND OF THE INVENTION

Silicon (Si) is one of most common elements in the universe by mass andit most exists in compound forms of sands, dusts, silicon dioxide(silica), or silicates, etc. Pure crystal silicon is a gray color andbrittle in nature. Silicon has atomic number 14, an atomic weight of28.09, and a density of 2.33 g/cm³. Silicon has a diamond cubic crystalstructure at room temperature, a melting point of 1310° C., a boilingpoint of 2357° C. Furthermore, silicon has a linear coefficient ofthermal expansion (CTE) of 2.49 μm/m-° C. at temperature 25.0° C.

Silicon is a semiconductor and plays a key role in the modern worldeconomy. In fact, the entire modern semiconductor microelectronicsindustry is established on silicon base. Silicon is widely used forintegrated circuits, chips, logic electronics devices, and memoryelectronics devices. Silicon and its compounds are not only used forforming the substrate where the semiconductor chips are built on butalso for the function units or layer for transistors and stackingstructures such as silicon electrode, silicon dioxide dielectric layers,and silicon nitride mask layers, inside the chips and integratedcircuits (IC).

In recent years, high-k metal gate (HKMG) transistor technology has beendeveloped and applied to 45 nm and below to manufacture IC devices for awide variety of high performance and low power applications such asgraphics, networking, and wireless mobile applications. One of the keytasks for high-k metal gate (HKMG) is to find out suitable new metalmaterials and reliable film formation methods to control of the channelwith high on currents and low leakage current and keep the integrity ofcomplex structures. Hf, TiAl, TiN, TiAlN, TiSiN, TaAlN, TaSiN, and rareearth metals are reported to be used as metal gate materials. Recently,much attention has been paid to utilize silicon to as metal gatestructure materials due to its unique chemical and physical propertiesand natural connection to the semiconductor technologies built on thesilicon.

Silicon and silicon nitride layers beyond the substrate inside the ICand chips are traditionally formed through chemical vapor deposition(CVD) method. As the microelectronics industry drives theminiaturization of devices and circuits towards nanometer dimensionutilizing 45 nm and below technologies, ever-increasing stringentdemands have been placed on the precision and minimum impact deviceintegrity of film/structure formation methods. Sputtering is a mechanismby which atoms are removed from the surface of a material (target) as aresult of collision with high-energy ions through a physical vapordeposition (PVD) technique wherein atoms or molecules are ejected from atarget material by high-energy ions bombardment so that the ejectedatoms or atom clusters can condense on a substrate as a thin film.Sputtering has more precise control of the transportation and depositionof mass atoms and has less thermal impact on the deposited filmstructure than a CVD process. Si sputtering target through a PVD processis becoming more widely used to silicon layers such metal gate electrodeand its compound structure layers such as silicon nitride and siliconcarbides inside microelectronics integrated circuits and communicationdevices for a wide variety of high performance and low powerapplications such as graphics, networking, and wireless mobileapplications.

SUMMARY OF THE INVENTION

In one embodiment, a silicon target comprises a Si target blank with adefect-free surface and molybdenum backing plate, molybdenum coppercomposite backing plate, or titanium aluminum composite backing plate.The target blank is bonded to backing plate through solder bonded, brazebonded, foil bonded, or other low temperature bonding methods. A surfaceprocess has been developed to achieve a nearly defect/damage-freesurface for silicon sputtering target to reduce the burn-in time andparticles on the films deposited from the sputtering targets. Thesurface process comprises the steps of machining, grinding, inspection,lapping, cleaning, surface damage removal (i.e., scratch removal),cleaning, polishing, cleaning, inspection, final cleaning, and finalinspection. This invention also provides methods of manufacturingsilicon sputtering targets.

BRIEF DESCRIPTION OF THE DRAWINGS

The below detailed description makes reference to the accompanyingfigures, in which:

FIG. 1 is process flow chart of a comparative Si target surface processA.

FIG. 2 is process flow chart of a Si target surface process inaccordance with one embodiment of the invention, namely process B.

FIG. 3 is a group of optical micrographs of the surfaces of Si targetsmade by comparative process A. Defects including scratches,micro-cracks, indentations, pores, and chips were observed on thesesurfaces.

FIG. 4 is a group of SEM images of the surfaces of Si targets made bycomparative process A. Defects including scratches, stains, pores,chips, and micro-cracks were observed on these surfaces.

FIG. 5 is a group of optical micrographs of the surfaces of Si targetsmade by process B in accordance with one embodiment of the invention.These surfaces are clean and almost defect-free except for few spotdefects.

FIG. 6 is interferometer imaging and surface roughness measurementresults of the Si target made by comparative surface process A. Parallelscratches were detected at target center. Interferometer roughness hasbeen performed at five positions—target surface center (PC), 3 o'clockposition (P3), 6 o'clock position (P6), 9 o'clock position (P9), and 12o'clock position (P12) 3 inches away from target surface perimeter. Themeasured roughness is PC=20.848 Angstrom, P3=16.749 Angstrom, P6=14.689Angstrom, P9=16.199 Angstrom, and P12=15.487 Angstrom. The averagemeasured roughness is 16.794 Angstrom and a standard deviation of 2.394Angstrom.

FIG. 7 is interferometer imaging and surface roughness measurementresults of the Si target made by surface process B in accordance withone embodiment of the invention. No apparent defects were detected onthe surfaces. The measured roughness is PC=11.911 Angstrom, P3=13.857Angstrom, P6=15.079 Angstrom, P9=14.625 Angstrom, and P12=11.888Angstrom. The average measured roughness is 13.472 Angstrom and astandard deviation of 1.500 Angstrom.

FIG. 8 is particle map and particle counts of Si films deposited fromthe Si target made by comparative process A. A total of 869 particleshave been detected on the films after the Si target had beenconditioned/burned in for more than 24 hours.

FIG. 9 is particle map and particle counts of Si films deposited fromthe Si target made by process B in accordance with one embodiment of theinvention. Only 25 particles have been detected on the films after theSi target had been conditioned/burned in for less than 8 hours.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The silicon sputtering target encompassed by this invention can have anysuitable geometry. The present invention includes a method ofmanufacturing the silicon target with a scratch removal surfacetreatment and good particle and burn-in performance. The silicon targethas a purity of at least 99.999% and preferably 99.9999% excludingdopants. The silicon target blank has diameter up to 550 mm and can beintrinsic, p-type doped, or n-type doped. The silicon blank can havepolycrystalline, single crystal, or semi-single crystal structure.Although other structures of Si can be used, the single crystalstructure is preferable according to this patent. The silicon rawmaterial and its dopant elements are melted to grow a silicon ingot froma molten liquid preferably through Czochralski crystal growth method orcasting process. The resulting ingot can have any size and any suitableshapes including round, square, and rectangular. The silicon ingot issubsequently inspected and wire sawed into ingot slices of differentdesirable thicknesses for making different parts. The silicon ingotslices undergo a surface process treatment to achieve desirabledimension and surface condition. The surface process includes but is notlimited to manufacturing operations such as machining, grinding,lapping, polishing, surface damage removal (i.e., scratch removal),etching, cleaning, and inspection, preferably additional cleaning stepswould be applied between these any two these operations, to form a Sitarget blank with desirable surface conditions and characteristics andgeometry dimensions. The finished Si blank is then bonded to a backingplate through solder bond or other low temperature bond methods toobtain a silicon target blank-backing plate assembly. The solder can bebut not limited to indium, tin-silver, and also can use nanofoil. Thereare many backing plate materials options include but are not limited toMo, Ti, Zr, Ta, Hf, Nb W, Cu and the alloys of these elements orcomposite backing plates like Mo/Cu and Ti/Al. The backing platematerials have a coefficient of thermal expansion (CTE) that is chosento closely match silicon's CTE. The bonded target assemblies can havediameters of 200 mm, 300 mm, and more and can be made for targetconfigurations with different OEM designs.

An exemplary silicon target with superfine surface has been producedaccording to the processes described above. Si raw material of 99.9999%or higher purity is used to grow an Si ingot, preferabledislocation-free single crystal Si ingot with a diameter from 300 mm to800 mm, preferable diameter of greater than 450 mm by use of theCzochralski crystal growth method. The composition of the resultingingot measured by the GDMS method is listed in the Table 1.

TABLE 1 Impurity element Content (ppm) Impurity element Content (ppm) S0.092 Ge 0.05 Li 0.005 As 0.005 Be 0.005 Rb 0.05 B 0.019 Zr 0.02 F 0.05Mo 0.02 Na 0.01 Pd 0.02 Mg 0.01 Ag 0.01 Al 0.07 Cd 0.03 P 0.01 In 0.02Cl 0.48 Sn 0.05 K 0.025 Sb 0.01 Ca 0.05 Cs 0.005 Ti 0.002 Ce 0.02 V0.002 W 0.02 Cr 0.01 Pt 0.01 Mn 0.005 Au 0.02 Fe 0.065 Hg 0.01 Co 0.005Tl 0.005 Ni 0.02 Pb 0.02 Cu 0.051 Th 0.001 Zn 0.05 U 0.001 Ga 0.01 C1.54 O 2.03 N 26.5

The weight concentration unit is ppm for all elements and total metallicimpurities contents are less than 1 ppm. The ingot is sawed into ingotslices of desirable thickness. The ingot slices are subsequentlysubjected to process A to achieve finish surfaces and dimensions.Comparative process A as herein referred to consists of manufacturingoperation steps machining, grinding, lapping, polishing, cleaning, andfinal inspection. Process B in accordance with one aspect of theinvention consists of manufacturing operation steps machining, grinding,inspection, lapping, cleaning, surface damage removal, cleaning,polishing, cleaning, inspection, final cleaning, and final inspection.The cleaning agents include but are not limited to de-ionized water andor acids. Further cleaning agents and cleaning apparatii may be chosenfrom U.S. Pat. No. 8,227,394 (Zhu et al.) herein incorporated byreference. As is stated in the '394 patent, a cleaning fluid, such as agas or liquid, can be prepared that may include a buffering agent andone or more polymers. The polymers are solubilized in the cleaning fluidand have a molecular weight of greater than about 10,000 g/mol. Thepolymers may, for example, be chosen from polyacrylic acid (PAA),polyacrylamide (PAM), hydroxyethyl cellulose (HEC). Copolymers of PAAand PAM are also mentioned as being exemplary. The weight percent of thepolymers in the cleaning fluid may be about 0.001-10% by weight.

In addition to water, other polar solvents such as isopropyl alcohol(IPA), dimethyl sulfoxide (DMSO), and dimethyl formamide (DMF) may bementioned. Weak acids and weak bases such as citric acid and ammonium(NH₄OH) can be mentioned as exemplary buffering agents.

In accordance with one aspect of process B, the scratch removal stepcomprises polishing to result in a substrate having an average measuredroughness of 15.0 Angstroms or less.

The finished Si target blanks are bonded to backing plate through indiumsolder bond or low temperature bond methods to obtain a silicon targetblank-backing plate assembly. There are many backing plate materialsoptions include but are not limited to Mo, Ti, Zr, Ta, Hf, Nb, W, Cu andthe alloys of these elements or Tosoh patented composite backing plateslike Mo/Cu and Ti/Al. The materials have coefficient of thermalexpansion (CTE) close to match silicon's CTE. The bonded targetassemblies can have different 200 mm, 300 mm, and beyond targetconfigurations with different OEM designs. In this case the backingplate material is Mo/Cu composite and the dimension is 300 mm typetarget.

Surface microstructures examinations have been performed on the silicontargets subjected to surface process A and B. As shown in FIG. 3, theoptical microscopy imaging indicates Si targets subjected to comparativesurface process A have a significant amount of surface defects includingscratches, micro-cracks, indentations, pores, and chips. As shown inFIG. 4, scanning electron microscopy (SEM) confirms scratches, stains,pores, chips, and micro-cracks surfaces exist on the surfaces of thesilicon targets subjected to process A. By the contrast, as shown inFIG. 5, the surfaces of the silicon targets subjected to process B areclean and nearly defect-free.

Non-destructive interferometer surface roughness measurements have beenconducted to measure surface roughness of the silicon targets subjectedto surface processes A and B. FIG. 6 is the interferometer images androughness measurement data for the surface of a Si target subjected toprocess A. Roughness has been performed at 5 positions—target surfacecenter (PC), 3 o'clock position (P3), 6 o'clock position (P6), 9 o'clockposition (P9), and 12 o'clock position (P12) 3 inches away from targetsurface perimeter. The measurement indicated average roughness is 16.79Angstrom with a standard deviation of 2.394 Angstrom. Parallel typescratches were detected. FIG. 7 is the interferometer images androughness measurement data for the surface of a Si target subjected toprocess B. No defects were detected on the target surfaces. The measuredroughness is PC=11.911 Angstrom, P3=13.857 Angstrom, P6=15.079 Angstrom,P9=14.625 Angstrom, and P12=11.888 Angstrom. The average measuredroughness is 13.472 Angstrom and a standard deviation of 1.500 Angstrom.The Si target surfaces subjected to process B have fewer surface defectsand lower roughness than the Si target surfaces subjected to process A.

Sputtering tests have been performed on these two 300 mm Si targets madeby the process A and B. The sputtering process parameters wereidentical. The target were conditioned or burned in to get achievedesirable low particle counts. For the Si target subjected to process A,the initial particle count was more than 2000 and as shown in FIG. 8,after 24 hours burning in, there were still over 800 particles detectedon the films. In contrast, initial particle count was low for the filmsdeposited from the target subjected to process B, and as shown in FIG.9, the particle count level was brought down to 50 particles or lowerunder less than 8 hour burn-in. In some cases, the particle countsdetected on the films deposited from the Si target subjected to processB were less than 5, this is one of the lowest particle counts has beenever achieved for the films deposited from the sputtering targets ofdifferent materials. The Si targets subjected to process B havesignificantly lower particle counts and shorter burn-in times than theSi targets subjected to process A.

We discovered that the Si target burn-in time and particle performancewere correlated to the target surface conditions or processes. Wediscovered the Si targets subjected to a surface process (Process B)consisting of surface removal, i.e., scratch removal, and multiplecleaning manufacturing operations have fewer surface defects and lowersurface roughness than the Si targets subjected to a surface process(Process A) without target surface damage removal and intermediatecleaning manufacturing operations. We believe process B remove Si targetsurface sub-layer damages and potential contaminations and thusresulting cleaner target surfaces and fewer surface defects, and fewerfilm particle counts and shorter burn-in times.

It is apparent then, that methods are disclosed herein for manufacturingsilicon sputtering targets. In one embodiment, a silicon sputteringtarget is provided in conjunction with a backing plate comprisingmolybdenum or a molybdenum copper composite. The silicon targetdiameters can range from about 800 mm or less.

The silicon blank has a high surface finish to achieve good sputterperformance and is made from a silicon ingot subjected to manufacturingoperations, such as sawing, machining, grinding, lapping, polishing,surface damage removal (i.e., scratch removal), etching, cleaning, andinspection. In preferred embodiments, additional cleaning and etchingsteps are provided between any two of these operations to form a silicontarget surface having desirable surface conditions. The resultingsilicon targets have a visually mirror shining reflection and nearlydamage/defect free surface, with a surface roughness of less than 500Angstroms, preferably less than 100 Angstroms.

Silicon targets in accordance with the invention have a short burningtime and the films deposited from the resulting Si targets have lowparticle counts.

The silicon target may, for instance, have a purity of at least about99.999% excluding dopants. The silicon target blank may have a diameterup to about 500 mm and can be intrinsic p-type doped, or n-type doped.The silicon blank can have polycrystalline, single crystal, orsemi-single crystal structure.

The backing plate can be pure molybdenum with a purity of 2N5 or higher,or in other embodiments, a molybdenum copper composite backing plate maybe utilized. In still other aspects of the invention, a titaniumaluminum composite may be used as the backing structure.

The target blank may be bonded to the backing plate through solderbonding such as by use of indium, tin-silver, and nanofoil. In otherembodiments, the backing plate materials have a coefficient of thermalexpansion that is chosen to closely match the coefficient of thermalexpansion of the silicon target material. Such backing plate materialsinclude, but are not limited to, No, Ti, Zr, Ta, Hf, Nb, W, Cu andalloys of these elements, especially Mo/Cu and Ti/Al composite backingplates.

The present invention has been disclosed in connection with thepreferred embodiments thereof, it should be understood that theinvention is not limited to the specific embodiments described since themeans herein comprise preferred forms of putting the invention intoeffect, and other embodiments may be within the scope of the inventionas defined by the following claims.

What is claimed is:
 1. Method of cleaning a silicon target comprisingproviding a Si blank, removing scratches from a surface of said blank soas to provide a mirror surface on said blank having a surface roughnessof 15.0 Angstroms or less, said method further comprising a first andsecond cleaning step, said first cleaning step being performed beforesaid scratch removal and said second step being performed after saidscratch removal, wherein either said first or second cleaning stepscomprise contacting said blank surface with a liquid cleaning solutioncomprising a polymer.
 2. Method of cleaning a silicon target comprisingproviding a Si blank, removing scratches from a surface of said blank soas to provide a mirror surface on said blank having a surface roughnessof 15.0 Angstroms or less, said method further comprising a first andsecond cleaning step, said first cleaning step being performed beforesaid scratch removal and said second step being performed after saidscratch removal, wherein either said first or second cleaning stepscomprise contacting said blank surface with a liquid cleaning solutioncomprising a polymer, wherein said polymer is a member selected frompolyacrylic acid, polyacrylamide and hydroxyethylcellulose.